He couldn't believe it at first, so the most he thought about it, the more he decided it must be true.

Options

A. but the more
B. No substitution required
C. but a most
D. although the more

Answer

A

Explanation

the more__, the more__ is a popular pair of phrases such as .the more you work the more you get paid.. It means the second condition is dependent on the first. However, the use of but is irrelevant because it shows contrast. Hence, the correct sentence structure: the more he thought about it the more he believed it to be true.